You sign up by clicking on the logo and then add any unwanted books you have (kids or adults) and when someone asks for them you get points. You get points for putting up your first books as well as points when people ask for your books. I love it, I get rid of unwanted books and get all kinds of books that I do want!
Another place I find cheap books is Salvation Army stores, other thrift stores and yard sales.
One caution, when getting books a little one might chew on either wash it if washable or use a disinfecting spray made for kids toys etc.
For more frugal ideas go to Biblical Womanhood and follow the links. I would recommend looking over her site as you do, she has good ideas!